How to remove the effect reflection of shadow
This is because you appear to have one light source and no bounces/indirect lighting.
So the only faces to receive any light will have to be facing the light source.

In the real world light is reflected and transmitted everywhere, resulting in almost everything getting illuminated at least a little bit.

You have several options to simulate this:


Increase the number of bounces in Render settings > Light paths, allowing light to reflect off surfaces and illuminate "around corners" in a physically correct fashion:


Enable ambient occlusion or add some ambient environment lighting:

In the real world there is always ambient light coming from the sky. Even with indoor renders this can make a difference (light coming through windows etc.)



Note that AO is an approximation of actual environment light, but as it's all based on distance it will illuminate things even inside closed shapes where environment light cannot normally reach.
Add more light sources with different sizes:

If you still want to stick with 0 bounces and no global illumination, try adding a few more light sources. Vary the strength and size to prevent it from becoming too "flat":